For shed-forming, in weaving machines heald shafts are used which carry healds and with which the warp threads are, by means of the healds, moved out of the middle plane of the warp threads. The heald shafts, with an upper shaft rod positioned above and a lower shaft rod positioned under the warp threads, extend over the entire width of the woven fabric to be manufactured. The ends of the shaft rods are interconnected by side binders. The shaft rods support respective shaft staves which, in turn, carry the healds. Shed-forming occurs by rapid upward and downward motion of the heald shaft. Since such upward and downward motion is derived from the rpm of the main drive shaft of the weaving machine, considerations about an increase of the speed of the reciprocating motion of the heald shaft also involve an increase of the machine rpm. It is a desideratum to increase the rpm of the weaving machine which, particularly in case of large fabric widths, leads to large loads on the heald shafts. The shaft rods are critical as concerns the productivity of the heald shaft. The bending resistance of the shaft rods during dynamically changing loads is of prime importance. The more rigid the shaft rods, the more rpm-resistant the entire heald shaft.
The majority of the manufactured and sold heald shafts is under a significant price pressure. For this reason, aluminum has been found very advantageous as the material for the shaft rods. On the one hand, aluminum components with complex cross sections may be made with extrusion presses and, on the other hand, a heald shaft made in this manner is, based on the low specific weight of aluminum, of remarkably low mass, yet relatively inexpensive. In cases where the load limit of aluminum is exceeded, other materials have also been successfully used.
For example, German Patent Document DE 37 02 524 describes a shaft rod which is a welded sheet metal construction. It constitutes a rectangular hollow profile member having an upper ledge and a lower ledge as well as two thin sheet metal components which form the side walls and connect the upper and the lower ledges with one another. The resistance of such a shaft rod to alternating bending forces is, based on the material used, considered to be greater than that of an aluminum shaft rod.
U.S. Pat. No. 5,345,974 discloses a shaft rod which is made of a fiber composite material and which has a stiffness similar to that of a shaft rod made of metal elements, but is of lesser weight.
While according to the starting point in the above-named patent documents the shaft rod has a substantially constant cross section along its entire length, German Patent Document DE 199 17 791 C1 shows a shaft rod having a varying profile along its length. The shaft rod is a hollow profile member which has its greatest height approximately in its middle. The profile height tapers towards the ends. Such a configuration is obtained by providing a hollow metal member whose height is reduced at both ends by milling. The exposed openings, resulting from the cuts through chambers in the metal profile member, are closed off by a strip.
Such a profile member can be considered as being optimized as concerns static loads. It has been found, however, that under dynamic loads, despite the greater stiffness of the profile member in the middle, a significant bending of the shaft rod occurs.
It is therefore the object of the invention to provide a shaft rod which exhibits but a slight bending tendency under dynamic loads.